Many power assist steering systems are available for use in automotive vehicles. Some steering systems provide assist by using hydraulic power, electric power, or a combination thereof. The use of an auxiliary or backup power source in a power assist steering system is known in the art. In such systems, a primary power source is employed during normal operation of the steering system to drive a power assist motor. A backup power source is usually maintained in a standby mode and is enabled for operation in the event of a failure of the primary power source. Thus, should the primary power source fail, power assist remains available by way of the backup power source. Once the primary power source is restored to working order, the backup power source is placed back into the standby mode.
U.S. Pat. No. 3,280,557 to Sattavara discloses a hydraulic power assist steering system having a primary pump and an auxiliary pump. Pressurized fluid is used to drive a hydraulic motor which provides the power assist. The primary pump is driven by the vehicle engine and is the main source of pressurized fluid for the power assist motor. The auxiliary pump is driven by the vehicle engine or by a separate electric motor. The output pressure from the primary pump is monitored by a pressure sensor. When the primary pump output pressure is less than a predetermined value, the auxiliary pump is actuated. In such an event, power assist remains available, the pressurized fluid to drive the hydraulic power assist motor being supplied by the auxiliary pump.
European Patent Application No. 84112152.8, Publication No. 0137491, discloses a microcomputer-based, fail-safe, steering control system. Power assist is provided by an electric assist motor which is electrically connectable between the positive terminal of the vehicle battery and ground potential through a switching transistor. A relay switch has its electrical contacts connected in parallel across the transistor. The relay contacts are normally closed. During normal operation, the relay is energized so that the contacts are open. The transistor is switched between an OFF condition and an ON condition by the microcomputer to control the amount of current through the electric assist motor. Should the microcomputer become disconnected from the battery, the relay contacts close to provide a current path to electrical ground so as to permit continued operation of the electric assist motor.